spl: blk: use CONFIG_SPL_FS_LOAD_PAYLOAD_NAME

We should target to unify the code for different block devices in SPL to
reduce code size.

MMC, USB, SATA, and Semihosting use CONFIG_SPL_FS_LOAD_PAYLOAD_NAME to
indicate the filename to load.

NVMe uses CONFIG_SPL_PAYLOAD in spl_blk_load_image().

CONFIG_SPL_PAYLOAD is meant to define which binary to integrate into
u-boot-with-spl.bin. See commit
7550dbe3 ("spl: Add option SPL_PAYLOAD").

Change spl_blk_load_image() to use CONFIG_SPL_FS_LOAD_PAYLOAD_NAME.

Fixes: 8ce6a2e1 ("spl: blk: Support loading images from fs")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mayuresh Chitale <mchitale@ventanamicro.com>